1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, this invention relates to a leadframe for use in semiconductor device, composed of a plurality of stacked element leadframes, and to its production method which provides high assembling productivity. Furthermore, this invention relates to a plastic encapsulated semiconductor device comprising the above leadframe.
2. Description of the Prior Art
Leadframes used for plastic encapsulated semiconductor devices are each usually composed of leads for electrical connections, a tab for mounting a semiconductor chip on it, leads for supporting the tab, and an outer frame for supporting all of these elements, wherein all of these elements are formed in one sheet metal. In such a leadframe, only one layer is available for leads or a tab. Moreover, all these leads, tab and others must be formed using the same material. This results in restriction for semiconductor devices to achieve higher integration density or to improve their electrical, mechanical and thermal characteristics. A technology to solve this problem is disclosed in Japanese Patent Unexamined Publication No. 58-27347 and in "Proceedings of the 39th Electronic Components Conference", 1989, pp. 494-503. In this technology, respective parts of a leadframe are formed in a plurality of different thin metal sheets, then these sheets are placed one upon another and connected to each other to form one stacked leadframe or one plastic encapsulated semiconductor device.
The above referenced literatures do not describe any concrete method for combining plural element leadframes of thin metal sheets into one stacked leadframe. For the application to plastic encapsulated semiconductor devices having a large number of pins or small body size, precise positioning of the element leadframes is really required before combining them into one stacked leadframe. For this reason, it is more preferable to combine element leadframes in such a manner that the positioning is performed by using outer frames which will be cut away later from smaller main portions, compared to the manner that only small essential parts required for final plastic encapsulated semiconductor device are combined without using outer leadframes. For example, one method to achieve this is as follows: First, leads of plural element leadframes are bonded each other by brazing or electric resistance welding. Then, the outer frames of element leadframes except one leadframe is cut off. After that, plastic molding is carried out as in the conventional method of encapsulating semiconductor devices with plastic. However, this method needs an extra step of cutting off outer frames after bonding the element leadframes. Besides, in the case of plastic encapsulated semiconductor devices having a large number of pins whose pitch is narrow, it is difficult to insert an outer-frame cutting tool into spaces between leads.